Re: 86 GMC Sierra Classic 2500 questions
 

I don't know the numbers but some carbs have wires off them.
most replace these without wires,unless u need them for Etesting
at the price of gas these day I would service it

Re: 86 GMC Sierra Classic 2500 questions
 

Well, I checked all of the lines from the carb to the transmission vacuum modulator. Straightened out the rubber connector like you said. Took it for a test drive and it has absolutely no balls. It sounds beautiful sitting there idling. Put it in gear and idle drops a bit, not enough to stall.... Step on the gas and it "sounds" like it's a beast but it just has no power. Frustrating!

Any suggestions on what to look at next? I hear a lot about the EGR valve causing problems. Could that cause this much power loss?

The distributor cap, wires and plugs have not been changed since I got the truck 4 years ago and I have no idea when they were changed previously. Should I go ahead and give that a try?

Re: 86 GMC Sierra Classic 2500 questions
 

Excuse my ignorance but it appears that I do not have an EGR valve and never did.... Correct me if I'm wrong but in the attached picture where I made the red circle would be where the EGR should be?? The shiny screw is a hose that has been plugged. Am I completely missing the EGR?

Re: 86 GMC Sierra Classic 2500 questions
 

The aftermarket intake you have does not have an EGR and that is fairly common. It is likely your engine had one at one time. I still suggest reading the spark plugs and doing a compression test. Do you have catalytic converter on this engine?

Re: 86 GMC Sierra Classic 2500 questions
 

Most aftermarket intakes don't have an egr valve. So you had one from the factory but when that intake was installed. They removed it. Also from your pictures you have an open vacuum port on the front of your carb. If I remember right, it should be hooked to your pcv valve.

"The distributor cap, wires and plugs have not been changed since I got the truck 4 years ago and I have no idea when they were changed previously. Should I go ahead and give that a try?" = YES

Re: 86 GMC Sierra Classic 2500 questions
 

I do not believe there is a catalytic converter. The PCV is hose is going to a port below the fuel filter..... Is this incorrect? Damn I feel like a moron.

Re: 86 GMC Sierra Classic 2500 questions
 

I have an "85 "M" code 3/4 ton. Does not have EGR, cat., charcoal canister, and able to run leaded gas, manual transmission, all from the factory. Point is that its most likely motor causing my problem. Runs good at idle and under no load. Compression check showed 160-170 on seven holes but 90 on number 5. Backed off both valves and applied shop air to that cylinder and it blew out of the exhaust port. Applied air to a good cylinder and it moved the piston to the bottom of the stroke. A compression tester is fairly inexpensive and can give you a good idea of your engine condition.
The port below your fuel filter looks like its an adapter for a spread bore(Quadrajet) to a square bore manifold so that is not the factory PCV port. I will have to look at my '85 to see where it goes. It may be fine where it is but is it capped off at the original location?

Re: 86 GMC Sierra Classic 2500 questions
 

After looking at my '85 I see that you are plugged into the correct port and your adapter has no port.

Re: 86 GMC Sierra Classic 2500 questions
 

Either way this port needs to be capped, Or used.

Re: 86 GMC Sierra Classic 2500 questions
 

Thanks for the advice. There was never anything over that port since I've had the truck.

I'm also curious if someone can tell me what this black Casco box is. I cannot find any information on this thing online.

Re: 86 GMC Sierra Classic 2500 questions
 

Google search says its an "AIR injection valve control module". For '86 C/K 20,30 350M. Can't help you beyond that. Not something I remember dealing with.

Re: 86 GMC Sierra Classic 2500 questions
 

So on top of my other problems I still have leaky intake bolts. The other day I used Permatex High Heat Threadlocker per the advise of the guy at Advance Auto Parts, waited 24 hours as recommended. I took it for a drive and oil still pools up! Also noticed the dipstick came loose. The pressure gauge reads between 40 and 50 usually and I thought that was normal. Is this too high?

Any recommendations for the oil leaking through the bolts? Permatex High Heat didn't seem to work and I thought I used enough.

Re: 86 GMC Sierra Classic 2500 questions
 

Check your valve cover bolts and gaskets.

Re: 86 GMC Sierra Classic 2500 questions
 

Thanks jetmech. I guess I just assumed oil was leaking through the intake manifold bolts.

Thanks to this site I have solved many of my problems. Finally got my instrument cluster lights to work along with the exterior lights except the tail lights will not come on when I turn on the headlights. All other lights are doing their job. All fuses have been replaced and wires checked. I'm thinking the switch might need to be replaced. Anything I else I should check before replacing the switch?

Also on my fuse box I have a port for ECMB and ECMI but no connections for a fuse to stick in there. Is this a problem? All of the fuses in the pic have been replaced and put in properly. I don't know why some were upside down and the improper fuse was in the radio port.

Re: 86 GMC Sierra Classic 2500 questions
 

On your truck the black box with the Casco on it was what passed for the "ECU/ECM" (computer). It had only one function. As jetmech85 suggests, it controlled the A.I.R. system diverter valves under certain conditions. It had two inputs;

1. Engine speed - it used the white wire that is lead out of the distributor cap (the same wire is used to drive a tachometer)

2. Incoming combustion air temperature - measured by a sensor that mounted through the side of the air cleaner.

Since your A.I.R system as been stripped, it can be removed.

On your tail light issue - just be sure that the correct bulbs are installed. You'd be surprised how often a single filament bulb gets jammed in to that dual filament socket.

Are your license plate lamps and rear markers working? If they are, that would indicate that power is available in the rear lighting wire harness on the brown wire (feeds the plate, rear markers & rear parking) and the problem is within the dual filament socket/bulbs.

Re: 86 GMC Sierra Classic 2500 questions
 

Put the proper amp fuse in the radio slot. Being upside down makes no difference. I have some empty slots on my Suburban also since there is no computer. You have most lights working so probably not the switch. Follow chengny's suggestions and also you may want to buy a test light.

Re: 86 GMC Sierra Classic 2500 questions
 

That picture of the fuse box was old, all of the fuses have been replaced with the proper ones. All of the rear bulbs have been replaced with the proper bulbs. The only ones that have not been replaced are the license plate bulbs. I also found a bulb up under the rear bumper which I have no idea what it is for. It is not connected to anything. In the picture you will see the rear wiring. Under the electrical tape there is a brown wire that is connected to the two license plate bulbs. The license plate bulbs and the taillights still do not work, however brakes turn signals and everything else work properly.

I am also working on brakes. As I was bleeding the right rear, one of the rear brake lines was badly rusted and leaked. I took it out and brought it to the local auto parts store. They gave me a new piece of brake line and a tube bender. But I didn't realize that the fittings were different sizes at each end, of course. I think one of these pictures is what you call a junction box? This junction box is the one with the larger fitting. Do you think the auto parts store will have brake line with two different size fittings at each end? Or does anybody have a suggestion on how to reuse the original fitting?

Re: 86 GMC Sierra Classic 2500 questions
 

My phone would not let me upload more than one picture at a time for some reason so here are the brake pictures. The line fitting from the auto parts store fits this end.

Re: 86 GMC Sierra Classic 2500 questions
 

The fitting at this junction is much larger.

Re: 86 GMC Sierra Classic 2500 questions
 

Get an adapter or you can use that piece if you cut then reflare the line.

Re: 86 GMC Sierra Classic 2500 questions
 

that is a rudimentary ABS system. designed to prevent wheel lock-up with a empty or lite load. just by pass it so you only have one size fitting. there notorious for jamming, hanging up, etc. i got rid of mine & the brakes feel a lot better.

Re: 86 GMC Sierra Classic 2500 questions
 

You were absolutely right. I bypassed it and now the rear brakes work perfectly.I just wish I had known that before wasting several hours on it.

Re: 86 GMC Sierra Classic 2500 questions
 

http://s5d1.turboimagehost.com/t1/20489986_vlv.jpg

This is called a "height sensing valve" and is part of the load compensating system. It works together with the proprtioning/compensating valve as follows:

These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve.
The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.
The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up.
The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.


It was a bad idea. GM gave up on it within a couple of years. This TSB was issued in 1988:

Brakes - Rear Brake Sensitivity

GMC NUMBER: 88-T-151

GROUP: 5 - Brakes

DATE: August, 1988

CORPORATENUMBER: 865010R
SUBJECT: REAR BRAKE SENSITIVITY - HEIGHT SENSING VALVE REMOVAL

MODELS: 1984-86 C/K 3500 AND 1987-88 R/V 3500

THIS BULLETIN IS A REVISION OF TRUCK SERVICE BULLETIN 85-B-79, DATED MAY, 1985, UPDATING FOR MODEL YEARS 1984-1988.
1984-1986 C/K 3500 and 1987-1988 R/V 3500 series trucks are equipped with a rear suspension height sensing device that optimizes the brake proportioning valve setting for the load the vehicle is carrying. The height sensing system adjusts the brake balance in vehicles subjected to a range of loading conditions. Occasionally, vehicle modifications by body builders or owners influence the height sensing device. If a vehicle has had rear suspension modifications that affect its trim height or the spring rate, and the driver comments about the brake modulation characteristics of the vehicle, it is recommended that the height sensing system be removed and the brake system be revised as described below:

Re: 86 GMC Sierra Classic 2500 questions
 

On your parking/marker/plate lighting issue. Here is a good way to start the diagnostic;

All of these lamps are supplied with power on the brown lead which is one of four circuits that make up the rear lighting harness. The other three are:

Back up lights - light green
Left Directional/brake - yellow
Right directional/brake - dark green

Schematically, the rear lighting harness looks like this (diagram compliments of chuckschevytruckpages):

http://s5d1.turboimagehost.com/t1/20...ightwiring.jpg

Here is what the actual harness looks like when layed out on the floor;

http://s5d4.turboimagehost.com/t1/20490242_HPIM1788.jpg


Climb under the rear end and locate the weather-tite style connector as shown below:

http://s5d4.turboimagehost.com/t1/20490244_HPIM1791.jpg

Separate the connector and - with the headlight switch in the parking light position - check for voltage on the brown lead at the cab side of the connector.

If you get power at the brown lead (on the cab side of the connector) - the problem is in the rear lighting harness. If not, the problem is in the forward side.

Do your courtesy (underdash) lights work when you open the door? How about your dome light? These lights are on the same circuit, so if they work, the problem is in the H/L switch or the wiring between the H/L switch and the weather-tite plug at the rear harness connection.

Re: 86 GMC Sierra Classic 2500 questions
 

Thank you chengny. I will thoroughly inspect the rear wiring later today. I don't know if mine is different thatn the pictures you posted but when I was under there before I did not see the connector you are talking about.

In the picture where the red circle is there is a brown wire that had been soldered to only the license plate bulbs (under the electric tape). One black wire to each bulb soldered to the brown wire.

The yellow circle is a loose connector that runs to a bulb that is under the bumper. What is this bulb and does it matter that it is not connected to anything? I don't have a picture of that right now.

When I was under there I did not see the "weather-tite style connector" that you are talking about but I will take a closer look. Thanks for all of your valuable information.

The dome light and courtesy lights work. However the instrument cluster lights work intermittently, Sometimes if I jiggle the switch just right the cluster lights will stay on.... I know, :waah:

One other thing I wanted to ask about the odometer. Mine is at 45,000 miles... Obviously it reached it's limit and started from zero again. It's probably more likely to be at 345,000 or more miles. How high do the odometers go on these trucks before rolling over to zero again?